The disclosed embodiment generally relates to a device and a method for transfer printing of an electrostatically charged toner image from an intermediate carrier of an electrographic printer or copier onto a recording mediums and fixing of the transfer-printed toner image onto the recording medium.
In electrographic printers or copiers, the transfer of a toner image from an intermediate carrier (for example a photoconductor drum or a photoconductor ribbon) onto a recording medium is designated as transfer printing. The section of the printing or copying device at which the intermediate carrier and the recording medium are brought into contact with one another is designated as a transfer printing region. In the transfer printing region, the intermediate carrier (meaning, for example, the generated surface of a photoconductor drum) and the recording medium move in the same direction with the same speed, while the toner is transferred from the intermediate carrier onto the recording medium. A print image of high quality can only be achieved on the recording medium when a uniform contact between recording medium and intermediate carrier is produced in the transfer printing region and when the recording medium and the intermediate carrier actually move with exactly the same speed in the transfer printing region.
In known printing or copying devices, the recording media are transported with transport rollers in the transfer printing region and effected on the side facing away from the intermediate carrier with a charge whose polarity sign is opposite to the charge of the toner image and of the intermediate carrier. The recording medium is thereby attracted by the intermediate carrier and transported through the transfer printing region adhering to this; at the same time the charge of the recording medium effects the transfer of the charged toner particles from the intermediate carrier onto the recording medium. Upon leaving the transfer printing region, the recording medium is then discharged with the aid of a discharge device with which it is loosened from the intermediate carrier and transported to a fixing device.
A transfer printing device of this type is known from WO 98/58297 A1. This transfer printing device has a contact element to press the recording medium onto the intermediate carrier. From WO 98/18052, a printer is known with two similar printing groups to which recording media are supplied via an input section. The printed recording medium are output via a common output section. An outlet channel is associated with the one printing group, via which outlet channel the recording media that have been printed on one side by this printing group can be re-supplied to this printing group for printing of the back side. A recording medium printed by the other printing group can be removed via the outlet channel to the output section by bypassing the transfer printing transport path of the first-cited printing group.
Further prior art is to be learned from the documents DE 199 56 505 A1, DE 43 24 148 C2, U.S. Pat. No. 5,666,622 A, US 2002/057933 A1, DE 40 39 158 A1, JP 2002-268 301 A, DE 77 36 767 U1 and DE 34 06 290 C2.
During the transport of a recording medium from the transfer printing region to the fixing device, its printed side may not be contacted because the not-yet-fixed toner image would otherwise be smeared. In conventional devices for transport of the printed recording medium, a vacuum table is therefore typically used in which the recording medium is held on a transport ribbon via suction pressure. In the fixing device, the recording medium is guided between two rollers whose generated surfaces abut closely to one another along a surface line and form a roller contact region or transport gap. The roller contact region or transport gap is also often designated in the German literature with the English term “nip”. Of the two rollers at least one is heated, and the toner image is affixed on the recording medium via pressure and heat.
Upon entrance of the recording medium into the roller contact region, the fixing rollers perform an additional milling task, whereby the recording medium is temporarily braked (this experiences a sudden jarring) that is in the direction opposite the transport direction. The distance between fixing rollers and transfer printing region is often less than the length of the longest recording medium to be printed in compactly designed printers or copying devices and in particular in devices with two printing groups. By the “length” of the recording medium, what is always meant in the following is the dimension of the recording medium in the transport direction, thus the length of the edges of the recording medium that are arranged parallel to the transport path. Given a rectangular recording medium, these do not necessary have to be the “lengthwise edges”, but rather can also be its transverse edges, namely when it is printed in the landscape format.
When the distance between the transfer printing region and the fixing rollers is shorter than the length of the recording medium, it can occur that the leading edge of the recording medium experiences a jarring in the roller contact region while the recording medium is still being printed at a rear section. In the event that this jarring transfer to the rear section, this leads to a smearing of the print image which is unacceptable.